Windows 10's interface for managing user accounts is practically unusable. Thankfully, the good ol' command line can save the day!

to add a new user account:
net users "NewUsername" "Password" /add /expires:never

to give it admin permission:
net localgroup administrators "Username" /add

to revoke admin permission
net localgroup administrators "Username" /delete

to delete a user account:
net users "Username" /delete

to change a users password
net users "Username" "NewPassword" /expires:never